This invention pertains to an improved bearing support for a flywheel power supply and more particularly an improved combination mechanical and magnetic bearing support system for a high-speed flywheel system that has significantly increased operating life capability, operates reliably and cleanly in vacuum, provides improved shock damage resistance, allows desirable low frequency rigid body resonances for transition to supercritical operation at reduced speeds, and is low in cost. Two general configurations of the invention exist; in one configuration the mechanical bearings are employed at all times but with very low loading due to off loading from the magnetic bearings, and in the other configuration the mechanical bearings are only employed intermittently because the magnetic bearings have full 5 axes stability.
Flywheel power supplies have emerged as an alternative to electrochemical batteries for storing energy with many advantages including higher reliability, longer life, lower or no maintenance, higher power capability and environmental friendliness. Flywheel power supplies store energy in a rotating flywheel that is supported by a low friction bearing system inside a chamber. The chamber is usually evacuated to reduce losses from aerodynamic drag. The flywheel is accelerated for storing energy and decelerated for retrieving energy through use of a motor/generator attached to the flywheel. Power electronics maintain the flow of energy in and out of the system and can instantaneously prevent power interruptions or alternatively manage peak loads.
To date, many types of support systems for flywheels have been developed. Some use mechanical bearings while others use magnetic bearings and yet others use a combination of the two types. Mechanical bearings are low in cost, however they suffer from fatigue and wear of the rolling elements and associated components. Magnetic bearings are usually more expensive and sometimes difficulties can arise in providing full five axes stable levitation in all operating conditions. To prevent damage to the power supply, mechanical auxiliary bearings are usually also provided in case the magnetic bearings fail to operate.
Because flywheel power supplies usually operate at very high speeds and are intended to last for ten to twenty years or more, careful design of the flywheel support is very important. Prior art has shown flywheel systems that can achieve long life with mechanical bearings by using mechanical bearings in combination with magnetic bearings. Ball bearings are used to provide radial support and the magnetic bearings greatly extend the ball bearing lives by providing some axial support, reducing the total load that they must carry. This support appears effective for smooth operation in low speed systems and where care can be taken to prevent damage to the bearings from shock loading during shipping and handling. However, to efficiently store large amounts of energy, it is desirable to further reduce radial loading, operating supercritically above the rigid body resonance. It is preferable to pass through the rigid body resonance at a relatively low speed such that the stored energy is low. The support must therefore be flexible. A flexible support also increases the durability and prevents damage to rolling element bearings from shock loading. Unfortunately, to achieve this increased energy storage, mechanical bearing life and robustness, the flexible support that must be employed becomes the life-limiting factor, and also can generate other problems including inadequate heat dissipation, plastic yielding, outgassing in the vacuum and deterioration. A flywheel that operates at 30,000 rpm for 20 years will experience an extremely large number of cycles, in excess of 300 billion cycles.
Full levitation magnetic bearing systems provide radial support flexibility and can in addition allow the flywheel to spin about its mass center at a relatively low speed. However, they also require an auxiliary mechanical support system to prevent total system destruction in the event of excessive loading and displacement. Such a mechanical support system must survive high shock loading, high frequency and vacuum operation, and must not deteriorate after years of residency in vacuum. The mechanical auxiliary support should also preferably survive one or more complete spin downs of the flywheel in the event that the magnetic bearings completely fail. Such spin downs can take hours depending on the energy storage of the flywheel.
This invention is an improved combination mechanical and magnetic support for a high-speed flywheel used in a flywheel power supply. Mechanical bearings provide radial support and partial axial support for the flywheel while one or more magnetic bearings significantly increase the life of the mechanical bearings by carrying at least 80% of the flywheel""s weight. Unlike prior art combination rolling element and magnetic support systems, the mechanical bearings are flexibly mounted. The radially flexible support provides the benefits of smooth transition to supercritical operation at low speed (preferably less than 30% of normal operating speed), lower radial bearing loading, and significantly improved shock damage prevention during operation and in shipping and handling. The flexible mount has a spring damper element constructed from metal mesh to enable it to match the increased life of the mechanical bearings. The metal mesh spring damper is uniquely capable of surviving the extreme cycles encountered, high frequency vibration and vacuum operation and long term residency. The mesh spring damper enables the long life of the high-speed combination bearing system by having both drastically improved life and properties over prior elastomeric and spring type flexible bearing supports.
A second configuration of the invention with the same benefits can also be employed with a full five axes stable magnetic bearing. In this case, the mechanical bearings are only in contact with the flywheel during excessive loading and displacements of the magnetic bearings. The mechanical bearings prevent damage during shipping and handling and during high-speed touchdowns. The use of a metal mesh spring damper again survives years of residency in vacuum without generating contamination and is capable of the impact and high frequency acceleration of a touchdown event. If the magnetic bearings fail completely, the mechanical support is capable of safely and reliably supporting the flywheel for spin down. As with the first configuration, the rigid body critical resonance is preferably pushed down below 30% of full speed such that transition occurs after 90% of the flywheels energy has already been removed. In both configurations of the invention, the support improves the durability, reliability and life of a flywheel power supply with a relatively low cost.